This invention relates to a method and apparatus for detecting respiratory distress conditions. More specifically, the subject invention provides a means for monitoring the output of a respiratory detector and differentiating between actual breaths and readings generated by the contractions of the patient's heart during respiratory arrest and using these readings to signal an alarm.
A variety of respiration monitors have been developed in the prior art for detecting the cessation of respiratory activity or apnea. One of the more common respiration monitors, an impedance pneumograph, measures the variations of the impedance in the thorax region of the body caused by movements of the chest during breathing. More specifically, as the patient breathes, and his chest alternately expands and contracts, the impedance of the thoracic cavity varies, and can be readily measured by existing electrical equipment. In a relatively simple embodiment of a prior art impedance pneumograph monitor, an alarm is connected to the device which is actuated when the variations of the output of the monitor cease, indicating respiratory arrest. In a typical intensive care situation, the sounding of the alarm alerts medical support personnel so that proper resuscitative procedures can be initiated.
The latter prior art respiration monitor, however, frequently fails to detect the onset of an apneic episode. More specifically, it has been found that the contractions of the patient's heart during respiratory arrest, often produces sufficient movement in the thoracic cavity to generate a varying output in the impedance pneumograph thereby preventing the associated alarm from being actuated. In order to obviate this shortcoming, various devices have been developed which utilize information from a cardiac monitor to increase the reliability of the device. For example, in U.S. Pat. No. 3,572,317 to Wade, an apnea monitor is disclosed which makes use of the clinical fact that during some apneic episodes, the heart rate tends to slow. The circuitry in Wade is designed to minitor both the heart and respiratory functions and to detect the situation where the heart rate is slowing. This information is correlated with output of the impedance pneumograph. More specifically, the circuitry in the Wade device is designed to detect the situation where both the contractions of the heart and the output of the impedance pneumograph are slowing at identical rates. When this situation exits it is likely that the output of the impedance pneumograph is merely a result of chest movements caused by the contractions of the heart rather than by actual breathing, and therefore an alarm will be sounded to indicate the apnea. One problem associated with the device disclosed in Wade is that a respiratory failure is not always immediately accompanied by a slowing of the heart rate. Thus, if a slowing of the heart rate does not occur relatively soon after a respiratory failure, the lack of oxygen in the body could result in damage prior to sounding of the alarm.
Another example of a prior art respiration monitor can be found in U.S. Pat. No. 3,976,052 to Juninger et al. The device disclosed in Junginger, similar to the Wade device, is concerned with the false breath-like output of the impedance pneumograph that is generated by the contractions of the patient's heart, and which would prevent the detection of an apneic episode. More specifically, the Junginger device utilizes that fact that during an apneic episode, the output of both the pneumograph and cardiograph, in terms of rate, will be substantially equal since the output of the former has been generated merely by the contractions of the heart. The circuitry in the Junginger device, therefore, compares the output received from the pneumograph and the cardiograph to determine if the heart rate and the apparent breathing rate are equal, and if such a situation exists, an alarm is sounded alerting the medical personnel. However, as can be appreciated, if the patient's breathing happens to coincide with his heart rate, the Junginger device might nevertheless indicate a stoppage of breathing, thereby disrupting both the patient and the medical staff. Thus, it would be desirable to provide an apnea monitor which is not solely dependent upon comparing the output of both a pneumograph and a cardiograph to determine if the frequencies of those outputs are equivalent. Further, it would be desirable to provide an apnea monitor which does not depend upon the slowing of the heart rate to signal the onset of respiratory failure.
As discussed more fully hereinbelow, the respiratory monitor of the subject invention satisfies these objects by combining the output produced by both an impedance pneumograph and an electrocardiograph in a novel manner. In contrast to the above described devices, the apparatus of the subject invention utilizes the clinical fact that during an apneic episode, the levels of oxygen in the body drop, while the levels of carbon dioxide rise. In response to this potentionally harmful situation, the forcefulness of the contractions of the patient's heart tends to increase in an attempt to increase blood circulation throughout the body to prevent the death of the cells due to lack of oxygen and excess carbon dioxide. The forcefulness of the contractions will continue to increase or will remain at a constant forceful level until the patient begins to breathe again or dies.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus and method for detecting respiratory distress which utilizes the fact that the force of contractions of the heart tends to increase during an apneic episode and which combines the information generated by an impedance pneumograph and an electrocardiograph in a novel manner, in order to signal the onset of respiratory distress.
It is a further object of the subject invention to provide an apnea monitor which may be used alone or in conjunction with other apnea devices to increase the effectiveness of the latter.
It is another object of the subject invention to provide an apnea monitor which is not dependent upon the slowing of the heart rate of the patient to produce a warning signal.
It is still a further object of the subject invention to provide an apnea monitor that detects the increasing amplitude of the output of an impedance pneumograph which is a result of the increasing force of contraction of the heart.
It is still another object of the subject invention to provide an apnea monitor which utilizes the electrical currents generated by the heart as a trigger for determining the amplitude of the analog wave form of the impedance pneumograph such that the forceful contractions of the heart which correspond to a respiratory distress condition are detected.